With the development of Internet Protocol (IP) technologies, global Internet users are growing and Internet routes are also growing. Because now Internet routes are mainly advertised using the Border Gateway Protocol (BGP), such growth imposes a new challenge to BGP. To tackle the new challenge, distributed BGP is designed so that BGP neighbor (that is peer) handlings are distributed in different BGP processes and each BGP process handles only peers related to it. In this way, a centralized handling is changed to a distributed handling.
Two types of BGP processes run on network devices in a distributed BGP system: Peer Distributed BGP (PD-BGP) process and Center BGP (C-BGP) process. There may be multiple PD-BGP processes and different PD-BGP processes handle different peers.
Because system load is related to the running state of the system (such as the number of routes, route flapping and policy), some PD-BGP processes may be subject to imbalance of loads; or with the growth of services, one PD-GBP process may become unable to bear the original peers within the BGP process. Both the above situations require the peers to be migrated from one PD-BGP process to another PD-BGP process, or from a PD-BGP process to a C-BGP process, or from a C-BGP process to a PD-BGP process.